Seasonal influenza infection is a major health concern for first-world and developing nations alike. Each year in the United States, five- to twenty-percent of the population gets the flu, more than 200,000 people are hospitalized from flu complications, and about 36,000 people die from flu. Worldwide, influenza causes tens of millions of respiratory illnesses and 250,000 to 500,000 deaths each year. New strains of avian influenza that are transmissible to humans are a critical concern for global health because these flu strains could yield pandemic disease for which no immunity exists, potentially resulting in millions of fatalities. “Avian flu” refers to a pathogenic avian influenza subtype that is highly contagious among birds and causes high mortality among domestic poultry. Outbreaks of avian flu among poultry and wild birds are ongoing in a number of countries, and at least three subgroups of avian flu viruses have infected humans to date. While avian flu infections of humans are rare, and most cases have been associated with direct poultry contact during outbreaks among livestock, infection in humans is very serious when it does occur: to date, over half of all reported human cases have been fatal. Since first reported in Hong Kong in 1996, the World Health Organization has carefully tracked avian flu and instances of animal-to-human influenza transmission, with confirmed cases reported from China, Indonesia, and Southeast Asia; Pakistan; Iraq; Egypt; and elsewhere, with 385 cases resulting in 243 deaths worldwide. While there is no evidence of sustained human-to-human transmission, instances of human-to-human spread of avian flu may have occurred. Since all influenza viruses have the ability to rapidly mutate, there is considerable concern that avian flu may be able to infect humans more easily and become communicable from one person to another. Also, avian flu virus strains have not infected many humans worldwide, so there is little or no immune protection against these strains in the human population; therefore, an influenza pandemic could easily occur if sustained avian flu virus transmission were to develop.
Three classes of influenza viruses, A, B and C, are responsible for human flu, with influenza A and B viruses causing seasonal epidemics of disease almost every winter. Influenza A viruses are divided into subtypes based on characteristics of two proteins, hemagglutinin (H) and neuraminidase (N), on the surface of the virus. There are 16 different hemagglutinin subtypes and 9 different neuraminidase subtypes, with H1N1 and H3N2 being the most common subtypes found in humans. The avian flu virus refers to influenza A H5N1. Influenza A is a negative-sense (3′ to 5′) single-stranded RNA virus. Its viral genome, which encodes 11 proteins (HA, NA, NP, M1, M2, NS1, NEP, PA, PB1, PB1-F2, PB2) in its RNA, cannot be translated into protein directly; rather, the virus depends on its RNA-dependent RNA polymerase to transcribe its genome to positive-sense RNA prior to translation. RNA-dependent RNA polymerases have no mammalian counterpart, which renders species selectivity less problematic in the development of therapeutics that target this enzyme. Other examples of viral RNA-dependent RNA polymerases include polioviral 3Dpol, vesicular stomatitis virus L, and hepatitis C virus NS5b; the latter is an active target for development of hepatitis C antiviral therapies. Unlike current flu targets (e.g., neuraminidase for Tamiflu), the influenza RNA polymerase is highly conserved and therefore less likely to suffer the resistance issues that current drugs face.
Recently, researchers reported the first atomic-resolution structural details of the influenza protein RNA polymerase, a critical enzyme for viral replication and a novel target for both therapeutic intervention and prophylaxis during influenza outbreaks (He, X., et al., Nature, 2008. 454: p. 1123-6; Obayashi, E., et al., Nature, 2008. 454: p. 1127-31). The influenza RNA-dependent RNA polymerase is a heterotrimer of three subunits, PA, PB1, and PB2, with the 310-helical N-terminal region of PB1 binding between the “jaws” of the PA protein. The PB1 helix is thought to be important for complex formation and nuclear transport and inhibits influenza A viral replication by interfering with polymerase activity. Recently, the PB2 subunit has also been shown to play an essential role in activity of the viral polymerase complex, for instance through contacts with the PB1 subunit. See Sugiyama et al, EMBO Journal, 2009, 28, 1803-1811. However, little is known about compounds capable of interfering with the binding and activity of these proteins. In general, there remains a need for therapeutic methods of treating viral diseases in which RNA-dependent RNA polymerases play a role, and for compositions and methods capable of modifying the activity such polymerases.